Regulating primary batteries efficiently for a long-life IoT device

I'm working on a long-life IoT sensor device which spends probably about 99.99% of its life asleep, waking up every 15 mins to take measurements and send data. I'm sure this is a problem that many people face as IoT becomes more popular.

I've figured out a few things to start: To get the longevity I require from the system the sleep mode current has to be tiny. Combined with this the primary battery has to be a high-capacity, low-self-discharge battery (Lithium Thionyl Chloride 3.6V battery, for example).

For example's sake, lets say the current requirements are typical of my typical wireless IoT device varying as follows, depending on the mode of operation over a period of 15 minutes:

Sleep mode: <10uA (99.99% of the time)

Measurement mode: 0.5-10mA (~10s)

Transmission mode: up to 50mA (2-5s)

Ideally I'd power the device straight from the battery, but a couple parts I'm using have a max input voltage of 3.6V, and a few LiSOCl2 batteries I've seen will sit at 3.7V before settling into their nominal voltage of 3.6V.

I'm aware the cell voltage may droop when under heavier load, so to be safe, 3V seems like a reasonable level to operate without being affected by the droop.

What is the best way to go about efficiently regulating this sort of primary battery to provide around 3V? The regulator would also have to consume very little current itself.